Vehicles may be fitted with evaporative emission control systems such as on-board refueling vapor recovery (ORVR) systems. Such systems capture and reduce release of vaporized hydrocarbons to the atmosphere, for example fuel vapors released from a vehicle gasoline tank during refueling. Specifically, the vaporized hydrocarbons (HCs) are stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the vapors. At a later time, when the engine is in operation, the evaporative emission control system allows the vapors to be purged into the engine intake manifold for use as fuel.
As such, for emissions test compliance, a boosted engine must be able to purge under boosted and naturally-aspirated conditions. Typically, engine intake manifold vacuum is used to purge the canister during non-boosted conditions. Therein, a purge valve coupled between the canister and the engine is opened so that fresh air can enter the canister, dislodge the trapped vapors, and direct the vapors to the intake manifold for combustion in the cylinders. An alternate purge path is used during boosted conditions. For example, as shown by Pursifull et al. in U.S. Pat. No. 8,312,765, a portion of boosted airflow is directed through an ejector (or venturi) and vacuum generated at the ejector is used to purge fuel vapors from the canister into the compressor, and from the compressor onwards to the boosted engine.
However, the inventors herein have recognized potential issues with such systems. As one example, the ejector may be limited by the amount of vacuum it can generate. During naturally-aspirated conditions, the amount of vacuum available may be significantly higher, allowing for larger purge rates. However, during boosted conditions, ejector choke may restrict the amount of vacuum that the ejector can produce. If the engine spends a large portion of the drive cycle under boost, the canister may not be sufficiently purged. To overcome the ejector and enable a higher purge rate, a substantially larger ejector may be required. However, this may add to component costs and packaging constraints. As another example, there may be conditions where the engine operates between boost and natural aspiration (herein also referred to as “no man's land”). During such conditions, there may neither be sufficient boost nor sufficient intake manifold vacuum for effectively purging the canister. Further still, the purge path under boosted conditions may be lengthy, affecting the purge rate. As such, if the canister is not sufficiently purged, engine emissions may be degraded.
In one example, the issues described above may be addressed by a method for an engine comprising: during boosted engine operation, purging a fuel vapor canister to a compressor inlet with positive pressure drawn from an exhaust-driven pump. In this way, a more thorough purging of a fuel system canister can be achieved during boosted engine operation without the need for additional ejector hardware. In addition, the purge pump may be used to enhance canister purging during selected naturally-aspirated conditions.
As one example, during boosted engine operation, engine exhaust gas may be used to drive a purge pump. Specifically, the discharged exhaust may be used to spin the pump, which then draws in fresh air and delivers it, at positive pressure, into the canister. Due to the pressurization, the air pumped into the canister may at a higher temperature than ambient air. An output of the pump may be controlled via adjustments to a wastegate coupled to the pump. The pumped fresh air dislodges fuel vapors trapped in the canister, and delivers them, via a dedicated purge path, to a compressor inlet. The purged vapors are then combusted in the engine. During un-boosted conditions, intake manifold vacuum may be applied on the canister and fuel vapors purged using ambient air may be delivered to the engine intake, downstream of an intake throttle, via an alternate purge path (distinct from the one used during boosted purge). However, during selected un-boosted conditions, where there is insufficient manifold vacuum available for purging the canister (such as during wide open throttle conditions), exhaust pressure from the un-boosted engine operation can also be advantageously used to drive the purge pump and clean the canister.
In this way, an exhaust-driven purge pump may be used to more completely purge a fuel vapor canister during boosted engine operation. Further, the exhaust-driven purge pump may be used to more completely purge a fuel vapor canister during un-boosted engine operation where there is insufficient manifold vacuum available for purging. The technical effect of using the purge pump to deliver pressurized ambient air into the canister is that the air entering the canister may be heated, serendipitously improving desorption of hydrocarbons from the canister. As such, this results in a cleaner canister. Further, by improving the purging capacity during boosted conditions, the need for relying on larger ejectors for effective boosted purging is reduced. Since operation of the purge pump does not rely on either the flow of boosted air from the compressor or intake manifold vacuum, boosted purging may not be affected by changes in boost pressure or engine intake vacuum. Instead, the purge pump may be advantageously used to allow canister purging to continue even as boost pressure or engine intake vacuum changes. By enabling the canister to be sufficiently purged, exhaust emissions compliance may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.